Cutting tools, in particular metal cutting tools, consist of a base body which is produced for example from hard metal, cermet, ceramics, steel or high-speed steel. To increase the tool life or to improve the cutting characteristics, a single-layer or multi-layer anti-wear protective coating made of hard materials is often applied to the base body by CVD or PVD processes. In PVD processes, a distinction is made between different process variants such as magnetron sputtering, arc evaporation (arc PVD), ion plating, electron beam evaporation and laser ablation. Magnetron sputtering and arc evaporation are counted among the PVD processes most frequently used for coating tools. Within individual PVD process variants, there are in turn various modifications, such as unpulsed or pulsed magnetron sputtering or unpulsed or pulsed arc evaporation, etc.
The target in the PVD process can consist of a pure metal or of a combination of two or more metals. If the target comprises a plurality of metals, then all these metals are simultaneously incorporated in the layer, built up in the PVD process, of a coating. The relative proportion of the metals to one another in the constructed layer will depend on the proportion of the metals in the target, but will also depend on the conditions in the PVD process, since some metals are released in greater quantities from the target under particular conditions and/or are deposited in greater quantities on the substrate compared with other metals.
To produce specific metal compounds, reactive gases are fed to the reaction chamber of the PVD process, such reactive gases being, for example, nitrogen for producing nitrides, oxygen for producing oxides, carbonaceous compounds for producing carbides or mixtures of these gases for producing corresponding mixed compounds, such as carbonitrides, oxycarbides, etc.
WO 96/23911 Al describes an anti-wear protective layer on a substrate, consisting of a coat of hard material applied directly to the substrate and a sequence of from 10 to 1000 further individual coats applied thereto, consisting alternately of a metallic hard material and a covalent hard material having a thickness of the individual layers of between 1 and 30 nm. The mechanical and chemical characteristics of the anti-wear protective layer are intended to be improved by the periodically alternating arrangement of individual coats of metallic hard materials and covalent hard materials.
WO 2006/041367 A1 describes a coated cutting tool consisting of a hard metal substrate and a coating which is deposited in the PVD process and comprises at least one coat of TiAlN having a thickness of 1.5 to 5 μm and a residual compressive stress of >4 to 6 GPa. The TiAlN coat is said to adhere more effectively to the substrate compared with known coats.
EP 2 298 954 Al describes a method for producing a coated cutting tool in which a hard material coating, for example TiAlN, TiAlCrN or TiAlCrSiN, is applied to a substrate by the PVD process, the bias voltage of the substrate being varied during the deposition process. The method is said to provide an improved wear resistance and a longer service life of the tool.
Particularly exacting demands are imposed on the tool in certain metalworking operations, such as milling and turning. Important parameters for tools of this type are a high degree of hardness, a high modulus of elasticity (E modulus, Young's modulus) and a low surface roughness. Known cutting tools for the described uses have a TiAlN coating which is applied in the PVD process and which typically has a modulus of elasticity of less than 400 GPa and a Vickers hardness of up to 3500 HV. When TiAlN layers of this type are deposited in the arc process, due to the low melting temperature of aluminium they tend to form so-called droplets on and in the layer which adversely affects the performance of the coating. A suitable choice of the parameters of the deposition process can increase the hardness and modulus of elasticity in the PVD process, but this generally leads to high residual compressive stresses in the layer of approximately >3 GPa which adversely affects the stability of the cutting edge. When subjected to a high stress, the cutting edge is liable to chip off at an early stage, thereby leading to rapid wear of the tool.